Rush hour is always a headache for drivers on the congested streets of the steamy towns of the oil-rich Gulf of Mexico region. But those caught in gridlock in Boca del Rio on the evening of Sept. 20 lived through an authentic nightmare. As commuters tried to get home from work, thugs bearing Kalashnikov rifles stepped out of six vans and ordered people to stay in their cars. The gangsters then parked two white trucks of the sort normally used to carry bananas or watermelons and opened their cargo doors. Some 35 corpses were rolled out onto the highway in front of horrified onlookers. The dead  many of whom had been tortured  included 12 women and two minors.

Mexican drug-cartel gunmen have been putting their victims on public display since the 1990s, but never before on the scale of the latest atrocity. Back in 2008, the world was shocked when thugs dumped 12 headless corpses in a pile in Yucatan state. Last January, the villains upped the ante when they displayed 15 bodies in the resort town of Acapulco. Rival cartels have begun trying to outdo one another in displays of depraved bloodletting. The sheer brazenness of the hit squads is especially remarkable. More corpses have been found in mass graves: more than 200 were dug up from pits in Durango in May. But when the gangsters stop the rush-hour commute with a grotesque display of carnage, the effect on public morale is terrifying. (See pictures of Mexico's ongoing drug war.)

In keeping with cartel tactics, the killers in Boca del Rio displayed messages  known as narcomantas  scrawled onto sheets draped on the sides of the trucks. "No more extortion. No more death of innocent people," said one. The messages alleged that the victims had been members of the Zetas, a gang led by former Mexican special-forces troops that has mushroomed into a criminal army stretching from the Texas border to Guatemala. The Zetas run kidnapping and extortion rackets on an industrial scale, shaking down everyone and everything from foreign migrants to casinos. The killers claimed they were exacting grim justice and were determined to exterminate this menace. "Don't let them extort you. Don't pay more protection payments," said one narcomanta.

Cartel gangsters' claiming to be vigilantes protecting the innocent is nothing new. In Sinaloa state, the cradle of Mexican trafficking, smugglers have long executed alleged kidnappers. In Michoacan, La Familia cartel thugs even claim to be Evangelical Christians delivering Old Testament justice. The same cartels also traffic huge amounts of narcotics, and their vigilante arguments are often cover for bumping off rivals. Often their victims are innocents. The Boca del Rio atrocity coincided with the arrival at a nearby hotel of judges and prosecutors from across Mexico for a meeting on the security crisis. (See TIME's photo-essay "A Mexican Meth Gang Wages a Holy Drug War.")

Authorities confirmed that at least some of the victims had criminal records. "We have identified some of the bodies and corroborated with our database that they all had criminal histories such as kidnapping, extortion, homicide, drug dealing and other crimes," said Reynaldo Escobar, attorney general for Veracruz state. Officials said they were also investigating whether some victims had been abducted from nearby prisons. Typical of Mexican violence, the hit squad that dumped the corpses escaped before police, soldiers and marines arrived to seal off the scene.

While his government condemned the incident, President Felipe Calderón was in New York City telling the U.N. General Assembly that "organized crime is killing more people and more youngsters than all the dictatorships put together". Since Calderón took office in December 2006, more than 40,000 people have died in drug-related violence. The President also struck out at the U.S. for failing to stop guns sold north of the border from reaching Mexican criminals and urged the U.N. to take on the problem. Tests show that a majority of the automatic rifles in the hands of Mexican criminals were purchased in U.S. stores. "For what reason do criminals have access to AK-47s, grenades and rocket launchers?" Calderón asked. "The U.N. has work to do on this."